Blood coagulation protects mammalian species when the integrity of the blood vessel wall is damaged and uncontrolled loss of blood threatens survival. Coagulation, resulting in the clotting of blood, is an important component of hemostasis. Under normal hemostatic circumstances, there is maintained an acute balance of clot formation and clot removal (fibrinolysis). The blood coagulation cascade involves the conversion of a variety of inactive enzymes (zymogens) into active enzymes, which ultimately convert the soluble plasma protein fibrinogen into an insoluble matrix of highly cross-linked fibrin. (See Davie, et al., "The Coagulation Cascade: Initiation, Maintenance and Regulation" Biochemistry 30:10363-10370 (1991)). Blood platelets which adhere to damaged blood vessels are activated and incorporated into the clot and thus play a major role in the initial formation and stabilization of hemostatic "plugs". In certain diseases of the cardiovascular system, deviations from normal hemostasis push the balance of clot formation and clot dissolution towards life-threatening thrombus formation when thrombi occlude blood flow in coronary vessels (myocardial infarctions) or limb and pulmonary veins (venous thrombosis). Although platelets and blood coagulation are both involved in thrombus formation, certain components of the coagulation cascade are primarily responsible for the amplification or acceleration of the processes involved in platelet aggregation and fibrin deposition.
A key enzyme in the coagulation cascade, as well as in hemostasis, is thrombin. Thrombin is intimately involved in the process of thrombus formation, but under normal circumstances can also play an anticoagulant role in hemostasis through its ability to convert protein C into activated protein C in a thrombomodulin-dependent manner. Thrombin plays a central role in thrombosis through its ability to catalyze the penultimate conversion of fibrinogen into fibrin and through its potent platelet activation activity. Direct or indirect inhibition of thrombin activity has been the focus of a variety of recent anticoagulant strategies as reviewed by Claeson "Synthetic Peptides and Peptidomimetics as Substrates and Inhibitors of Thrombin and Other Proteases in the Blood Coagulation System", Blood Coag. Fibrinol. 5:411-436 (1994). The major classes of anticoagulants currently used in the clinic directly or indirectly affect thrombin (i.e. heparins, low-molecular weight heparins and coumarins). Thrombin is generated at the convergence of the intrinsic and extrinsic coagulation pathways by the prothrombinase complex. The prothrombinase complex is formed when activated Factor X (factor Xa) and its non-enzymatic cofactor, factor Va assemble on phospholipid surfaces in a Ca.sup.+2 -dependent fashion as reviewed by Mann, et al., "Surface-Dependent Reactions of the Vitamin K-Dependent Enzymes", Blood 76:1-16 (1990). The prothrombinase complex converts the zymogen prothrombin into the active procoagulant thrombin.
The location of the prothrombinase complex at the convergence of the intrinsic and extrinsic coagulation pathways, and the significant amplification of thrombin generation (393,000-fold over uncomplexed factor Xa) mediated by the complex at a limited number of targeted catalytic units present at vascular lesion sites, suggests that inhibition of thrombin generation is an ideal method to block uncontrolled procoagulant activity. Unlike thrombin, which acts on a variety of protein substrates as well as at a specific receptor, factor Xa appears to have a single physiologic substrate, namely prothrombin.
Plasma contains an endogenous inhibitor of both the factor VIIa-tissue factor (TF) complex and factor Xa called tissue factor pathway inhibitor (TFPI). TFPI is a Kunitz-type protease inhibitor with three tandem Kunitz domains. TFPI inhibits the TF/fVIIa complex in a two-step mechanism which includes the initial interaction of the second Kunitz domain of TFPI with the active site of factor Xa, thereby inhibiting the proteolytic activity of factor Xa. The second step involves the inhibition of the TF/fVIIa complex by formation of a quaternary complex TF/fVIIa/TFPI/fXa as described by Girard, et al., "Functional Significance of the Kunitz-type Inhibitory Domains of Lipoprotein-associated Coagulation Inhibitor", Nature 338:518-520 (1989).
Polypeptides derived from hematophagous organisms have been reported which are highly potent and specific inhibitors of factor Xa. U.S. Pat. No. 4,588,587 awarded to Gasic, describes anticoagulant activity in the saliva of the Mexican leech, Haementeria officinalis. A principal component of this saliva is shown to be the polypeptide factor Xa inhibitor, antistasin, by Nutt, et al., "The Amino Acid Sequence of Antistasin, a Potent Inhibitor of Factor Xa Reveals a Repeated Internal Structure", J. Biol. Chem. 263:10162-10167 (1988).
Another potent and highly specific inhibitor of Factor Xa, tick anticoagulant peptide, has been isolated from the whole body extract of the soft tick Ornithidoros moubata, as reported by Waxman, et al., "Tick Anticoagulant Peptide (TAP) is a Novel Inhibitor of Blood Coagulation Factor Xa", Science 248:593-596 (1990).
Other polypeptide type inhibitors of factor Xa have been reported including the following citations by: Condra, et al., "Isolation and Structural Characterization of a Potent Inhibitor of Coagulation Factor Xa from the Leech Haementeria ghilianii", Thromb. Haemost. 61:437-441 (1989); Blankenship, et al., "Amino Acid Sequence of Ghilanten: Anti-coagulant-antimetastatic Principle of the South American Leech, Haementeria ghilianii", Biochem. Biophys. Res. Commun. 166:1384-1389 (1990); Brankamp, et al., "Ghilantens: Anticoagulants, Antimetastatic Proteins from the South American Leech Haementeria ghilianii", J. Lab. Clin. Med. 115:89-97 (1990); Jacobs, et al., "Isolation and Characterization of a Coagulation Factor Xa Inhibitor from Black Fly Salivary Glands", Thromb. Haemost. 64:235-238 (1990); Rigbi, et al., "Bovine Factor Xa Inhibiting Factor and Pharmaceutical Compositions Containing the Same", European Patent Application, 352,903 (1990); Cox, "Coagulation Factor X Inhibitor From the Hundred-pace Snake Deinagkistrodon acutus venom", Toxicon 31:1445-1457 (1993); Cappello, et al., "Ancylostoma Factor Xa Inhibitor: Partial Purification and its Identification as a Major Hookworm-derived Anticoagulant In Vitro", J. Infect. Dis. 167:1474-1477 (1993); Seymour, et al., "Ecotin is a Potent Anticoagulant and Reversible Tight-binding Inhibitor of Factor Xa", Biochemistry 33:3949-3958 (1994).
Factor Xa inhibitory compounds which are not large polypeptide-type inhibitors have also been reported including: Tidwell, et al., "Strategies for Anticoagulation With Synthetic Protease Inhibitors. Xa Inhibitors Versus Thrombin Inhibitors", Thromb. Res. 19:339-349 (1980); Turner, et al., "p-Amidino Esters as Irreversible Inhibitors of Factor IXa and Xa and Thrombin", Biochemistry 25:4929-4935 (1986); Hitomi, et al., "Inhibitory Effect of New Synthetic Protease Inhibitor (FUT-175) on the Coagulation System", Haemostasis 15:164-168 (1985); Sturzebecher, et al., "Synthetic Inhibitors of Bovine Factor Xa and Thrombin. Comparison of Their Anticoagulant Efficiency", Thromb. Res. 54:245-252 (1989); Kam, et al., "Mechanism Based Isocoumarin Inhibitors for Trypsin and Blood Coagulation Serine Proteases: New Anticoagulants", Biochemistry 27:2547-2557 (1988); Hauptmann, et al., "Comparison of the Anticoagulant and Antithrombotic Effects of Synthetic Thrombin and Factor Xa Inhibitors", Thromb. Haemost. 63:220-223 (1990); Miyadera, et al., Japanese Patent Application JP 6327488 (1994); Nagahara, et al., "Dibasic (Amidinoaryl)propanoic Acid Derivatives as Novel Blood Coagulation Factor Xa Inhibitors", J. Med. Chem. 37:1200-1207 (1994); Vlasuk, et al., "Inhibitors of Thrombosis", WO 93/15756; and Brunck, et al., "Novel Inhibitors of Factor Xa", WO 94/13693. Al-obeidi, et al., "Factor Xa Inhibitors", WO 95/29189, discloses pentapeptide X1-Y-I-R-X2 derivatives as factor Xa inhibitors. Said compounds are useful for inhibiting blood clotting in the treatment of thrombosis, stroke, and myocardial infarction.